1. Technical Field
The present invention relates to an athermal AWG module used as an optical multiplexer/demultiplexer, for example, in wavelength division multiplexing optical communications, which module is unaffected by the temperature, that is, athermal.
2. Related Arts
In an arrayed waveguide grating (AWG)-type optical multiplexer/demultiplexer that handles the function of an optical multiplexer/demultiplexer, the center wavelength (transmission center wavelength) is dependent on the temperature due to temperature dependence of the refractive index of the silica glass. In order to cancel its temperature dependence, Japanese Patent Laid-open Publication No. 2006-284632 discloses an AWG-type optical multiplexer/demultiplexer in which waveguides are partially cut to separate a chip and separated chips are connected with a compensation plate to achieve athermalization (temperature-independence).
By the way, in the case of manufacturing an athermal AWG module by accommodating the conventional AWG chip as disclosed in the above-mentioned JP 2006-284632 into a package, there is a problem that an adhesive agent for fixing the compensation plate to the separated chips is deteriorated by penetration of water and the center wavelength fluctuates. Further, as the waveguides are cut partially, there is a need to fill matching oil that is matched in refractive index to the waveguides, and any contrivance needs to be made for the package to prevent leakage of the oil. In order to solve these problems, there is contemplated an athermal AWG module that adopts a hermetic structure in which a cover for blocking an opening of the package is secured to the package by welding. The inner structure of this athermal AWG module is illustrated in FIG. 10 and the module in which the cover is closed for blocking the opening is illustrated in FIG. 11. This athermal AWG module 100 has an athermal AWG chip 120 as disclosed in the above-mentioned JP 2006-284632, a package 130 for accommodating the chip and a cover 150 blocking an opening 140 of the package 130. In the athermal AWG chip 120, an AWG 110 having partially cut waveguides is formed on a substrate. This athermal AWG chip 120 is divided into two by a crossing separation surface 300 and a noncrossing separation surface 310 and the separated chips are connected with the compensation plate 330. Then, in this athermal AWG module 100, matching oil is injected through an oil inlet (not shown) into the package 130 hermetically sealed by welding.
However, in this athermal AWG module 100, there are problems of high cost and larger module size for required welding margin. In other words, as there is local temperature increase in the welded part 160 during welding and the temperature of the chip also increases, there is a need to provide a predetermined distance A between the welded part 160 and the athermal AWG chip 120. This results in the problems that the module size becomes larger. Besides, as the welding operation has to be done, the cost becomes higher.
The present invention has been made in view of the conventional problems, and it is an object of the present invention to provide an athermal AWG module that is low in cost and small sized and has high reliability.